The present invention is directed to an extrusion die, a method of using the die to produce co-extruded polymeric articles, and co-extruded polymeric articles produced therewith having internal discontinuities, such as stripes.
Extruded polymeric webs are used in many applications, including the production of thin films for use as tape backings, medical films, and vapor barriers. Polymeric materials that are suitable for extrusion are often polyolefins such as polyethylene, polypropylene, and polybutylene; polyamides such as nylon; polyesters such as polyethylene terephthalate; or polyvinylidene fluoride. Although these polymeric materials and others are suitable for use in forming a polymeric web, they can have limiting characteristics that substantially narrow their suitable uses. For example, reinforced polypropylene webs often have very good tensile strength, but have less than desirable cross-web tear strength. Therefore, due to a propensity to tear too easily in the cross-web direction, oriented polypropylene webs do not traditionally make satisfactory products requiring cross-web strength, such as strapping tape products. Similarly, natural and synthetic rubber have excellent elasticity, but are difficult to fuse with most polymeric materials. Therefore, due to the challenge of creating a good bond between materials, rubber webs are difficult to use in products that require them to be joined with other polymeric materials.
A hybrid polymeric web combining two polymers is described in Krueger et al. (U.S. Pat. No. 5,429,856). An apparatus for making a co-extruded web is disclosed in Schrenk et al. (U.S. Pat. No. 3,485,912).
In certain embodiments of the invention, the polymeric co-extruded web comprises a plurality of uniform, distinct, phases (embedded phases) that are discontinuous in a cross-web direction. The embedded phases preferably have a width uniform to within a coefficient of variation of less than 8 percent for three consecutive discontinuous phases. The width of these embedded phases is measured in a cross-section of the co-extruded web cut transverse (i.e., cross-web) to the machine direction (i.e., down-web) and is the largest dimension of the cross-section of the embedded phases in the cross-web direction. The embedded phases are substantially continuous down-web and are surrounded by a matrix having two or more distinct layers of the same or different materials.
In one embodiment, the discontinuous phases of the polymeric co-extruded web are spaced at substantially uniform intervals in the cross-web direction. In another embodiment, the plurality of discontinuous phases consists of a first discontinuous phase positioned proximate a first edge of the web, and a second discontinuous phase positioned proximate a second edge of the web.
The present invention is also directed to an extrusion die for forming a polymeric co-extruded web. In specific embodiments, the die includes a body containing two chambers. An adjustable vane is positioned between the chambers. The adjustable vane is at least partially hollow, having a cavity within its interior. The vane has at least one opening (inlet) in the cavity positioned to receive a material being extruded, and at least two openings (outlets) in the cavity positioned in a tip to extrude material into the body of the die. The cavity inlets and outlets are sized so that the width of the embedded phases extruded from the vane outlets into the die body are uniform. This uniformity of the embedded phases is an advantage of this invention.
In operation, a first material is forced into the chambers of the body of the die, and a second material is forced into the cavity of the vane. The first material is conveyed through the chambers of the die, passing around and past the vane. The second material enters the cavity of the vane and subsequently flows out through the tip of the vane oriented toward the downstream end of the die and into the die body, where it flows along with the first material in laminar flow until the combined materials exit the die to form a co-extruded web.
The invention is further directed to a process of making a polymeric co-extruded web. The process includes providing an extrudable material and an extrusion die. In a specific embodiment, the die contains two chambers and an adjustable vane between the chambers. The vane contains a cavity having at least one input orifice positioned to receive extrudable material and at least two exit orifices. The cavity is designed so that the pressure drop of molten polymer within the cavity is significantly less than the pressure drop through the exit orifices to yield embedded phases of improved width uniformity over those extruded by known techniques. A first material is extruded through the chambers of the die, and a second material is extruded through the exit orifice in the vane to produce a co-extruded web containing the first and second extrudable materials. The second material is embedded between the two layers of the first material. Alternatively, different polymeric materials may pass through each die chamber to form two layers of different materials that are positioned around the embedded phase material.